The present invention relates to solutions of hydrogen silsesquioxane resin which are stable when stored for extended periods of time. These stable solutions are obtained by incorporating small amounts of acids in the solution.
Hydrogen silsesquioxane resin is known in the art to be useful as a precursor to silica-containing ceramic coatings. For instance, U.S. Pat. No. 4,756,977 granted to Haluska et al. describes a process for forming such a coating in which hydrogen silsesquioxane is diluted in a solvent, applied to a substrate and ceramified by heating.
Various methods for producing hydrogen silsesquioxane resin are also known in the art. For example, U.S. Pat. No. 3,615,272 describes a process for preparing this material which comprises adding trichlorosilane to a reaction medium comprising sulfuric acid and an aromatic hydrocarbon, washing the reaction mixture until neutral and recovering the condensed resin. Despite the fact that the reference teaches hydrolysis and condensation in an acid, the resultant product is neutralized to prevent gelation.
U.S. Pat. No. 5,010,159 teaches a similar process in which a hydridosilane is reacted in an arylsulfonic acid hydrolysis medium followed by separating the resultant resin and then contacting it with a neutralizing agent. Again, however, the resultant resin is neutralized to prevent gelation.
The shelf life of the above hydrogen silsesquioxane resin diluted in a solvent may, however, be variable due to the resin crosslinking and, thus, increasing in molecular weight. The present inventors have now discovered that the incorporation of an acid in the hydrogen silsesquioxane resin solution limits the changes in the molecular weight of the resin.